The appearance of a transparent product is defined by how much light will pass through it and how objects will appear through the transparent product. Translucent products, such as for example glass, transparent sheets and the like, are used in many fields. In this case the optical properties play an important role, depending upon the field of application. In this way for example, a high degree of transmission is required of glass panels and sheets which are used for greenhouses. A sheet used for packaging, on the other hand, should allow the contents to be recognized as clearly as possible and with as little clouding as possible.
Devices for examining these optical properties are known in the art. Generally, such devices combine a light source to trans-illuminate a sample and a detector to detect this light once it has passed through the sample. Various analyses are performed to evaluate the conditions of the interior volume and surface conditions of the sample material that contribute to its transparency.
The appearance of an object is composed of many elements relating to the light absorption and scattering properties of the material under review. Transmission haze (or wide-angle scattering of light through a trans-illuminated sample) is a necessary measurement quantity that must be measured in order to ensure a uniform and consistent product. Furthermore, the haze characteristic of a sample is useful in analyzing pertinent process parameters and material properties, e.g. cooling rate or compatibility of raw materials.
Haze is considered that percentage of light which, in passing through a material, is deviated from its original path by more than 2.5 degrees on the average. Thus, haze provides a lack of distinctness of an image and contributes to an overall lack of clarity in a material. One way of measuring the transmission haze of a sample is to use an integrating sphere. An integrating sphere is a type of diffuse reflectance device. Light is directed into the interior volume of the integrating sphere, which is coated with a matte surface. The diffusion of light in the integrating sphere proceeds according to Lambertian behavior. This is distinct from specular reflectance which occurs on mirror-like surfaces. In an integrating sphere light is scattered and re-scattered inside the sphere until it escapes the sphere through an open port or is absorbed by the interior surface of the sphere. This multiple reflection causes the intensity of the light inside the sphere to be relatively constant at all points inside the sphere.
The article entitled Standard Test Methods for Haze and Luminous Transmittance of Transparent Plastics, ASTM D1003-07 (2008) hereby incorporated by reference, describes traditional methods of determining haze and luminous transmittance in transparent plastics. For example, a haze meter device is described which incorporates an integrating sphere designed to determine the angular distribution of the diffused portions of the light. In this device, light that has passed through the sample is attenuated by light loss through a selected circular aperture, which at the sample subtends an angle of 5 degrees. Alternatively, a spectrophotometer with an included integrating sphere is also described as suitable for use in determining the haze characteristics of the sample under measurement. In either device described in the standard, light that first scatters in the integrating sphere and passes through the sample reaches the light detector only if it is scattered from outside a selected circular aperture, which at the sample subtends an angle of 5 degrees. In both of these instances, the prior art devices were configured to measure light depletion due to use of only one size of port to constrain the angle of scattering from the sample. As a result, only haze is measured and not low-angle scattering. Haze characteristics such as wide angle scattering, are not easily measured. Even for haze measurement alone, four measurements are required for any evaluation of transmission haze, which variously involve a reflectance standard in/out of position, a light trap in/out of position, and a specimen in/out of position (see Section 7.2.1 in D1003). These measurements comprise incident light, total light transmitted by the sample, light scattered by the instrument, and light scattered by the instrument and sample.
In addition to this described standard method, there is also a further standard method according to the ISO. In the case of this further standard method the intention is also to take into consideration errors which occur as a result of change in efficiency—caused by the samples—of the Ulbricht sphere (integrating sphere) used. In this case for example single-beam methods are used, in which the sample to be investigated is applied to two different outlets of the Ulbricht sphere. In addition, double-beam methods are known, in which two light bundles are used, one constituting the measurement bundle which passes through the sample, and a further bundle which does not pass through the sample but illuminates the inner wall of the Ulbricht sphere. The last-named method has the drawback, however, that the aforesaid light bundles must be precisely attuned to each other and, in addition, influences from the background illumination (for example illumination of the space) should also be taken into consideration.